Generally, a cold rolled steel strip is subjected to an annealing process in order to relieve strains generated thereon during the cold rolling process. In the case where the cold rolled steel strip is an electric steel strip containing 2 to 4% by weight of silicon, after the electric steel strip is formed into desired final dimensions by the cold rolling, an annealing is applied to the cold rolled electric steel strip for the purpose of adjusting the composition of the steel strip by means of, for example, decarburization and denitrification, or controlling the crystallization of the steel strip.
Generally, the annealing is carried out in a continuous annealing procedure, a box annealing procedure or a combination of the above-mentioned procedures.
In the case where the annealing process includes a box annealing procedure, a plurality of steel strips are individually wound around a winding core to provide a plurality of coils and each coil is placed on a base plate. A plurality of combinations, each consisting of a coil and a base plate, are piled up in a box annealing furnace in such a manner that the longitudinal axis of the winding core in each coil is vertical and, then, a box annealing procedure is applied to the coils.
In the steel strip coil which has been box annealed by the above-mentioned procedures, strains are produced in the side edge portion of the steel strip which has been kept in contact with the base plate during the box annealing procedure and the side edge portion is deformed. When the box annealed steel strip is an electric steel strip and the electric steel strip is used in a laminated state, for example, in the production of a transformer or a motor core, the strains produced in the side edge portion of the steel strip cause serious disadvantages. In the case where the box annealed steel strip is an ordinary cold rolled steel strip, the strains can be released by a temper rolling applied to the box annealed steel strip. However, the larger the strains in the box annealed steel strip, the more difficult to release the strains by means of the temper rolling. Therefore, even in the case of the ordinary cold rolled steel strip, it is desirable that the strains created in the box annealed steel strip be as small as possible. Especially, in the case of a cold rolled steel strip, particularly, an electric steel strip, to which a finishing operation, for example, a temper rolling operation, is not applied, the side edge portion of the steel strip in which the strains have been produced, is cut off. When no cutting off operation is applied to the side edge portion of the steel strip having the strains, it is necessary to apply a reforming procedure to the steel strip. However, the reforming procedure causes the magnetic flux density and other electromagnetic properties of the steel strip to be undesirably deteriorated. Therefore, it is desirable that the production of the strains in the steel strip during the box annealing procedure be prevented as much as possible.
It is known that, in order to decrease the production of the strains in the steel strip during the box annealing procedure, the following approaches have been attempted.
(1) A side edge portion of a steel strip is notched, the steel strip is formed into a coil and placed on a base plate in such a manner that the notched side edge portion is brought into contact with the base plate, and then, the steel strip coil on the base plate is placed in the box annealing furnace and box annealed therein.
(2) A steel strip is formed into a coil in such a manner that the side edges of the resultant coil become uneven, the steel strip coil is placed on a base plate in such a manner that an uneven side edge of the coil is brought into contact with the base plate and, then, the coil on the base plate is subjected to a box annealing procedure.
However, the above-mentioned approaches have failed to satisfactorily decrease the creation of strains in the side edge portion of the steel strip. Therefore, the above-mentioned approaches have not yet been practically utilized in industry.
In addition to the above-mentioned approaches, the following approaches have been attempted.
A. When a steel strip is wound to form a coil, an extremely large tension is evenly applied to the entire body of the steel strip. The coil is subjected to a box annealing procedure.
B. When it has been damaged during a winding procedure applied to a steel strip prior to the box annealing procedure that a side edge portion of the steel strip has been stretched so as to produce larger stresses therein than those in the other side edge portion of the steel strip, the resultant steel strip coil is placed on a base plate so that the side edge of the steel strip, which has been deemed to have larger stresses than that of the other side edge, is brought into contact with the base plate, and then, the coil on the base plate is subjected to a box annealing procedure.
The approach A is effective for reducing the production of strains in the side edge portion of the steel strip kept in contact with the base plate. However, the approach A is disadvantageous that sometimes undesirable thermal strains are produced in the middle portion of the steel strip, or during the box annealing procedure, the flow of the atmospheric gas in the box annealing furnace into the inside of the coil becomes unsatisfactory, and this unsatisfactory flow causes the adjustment of the composition of the steel strip and the chemical reactions on the surface of the steel strip, which are the purposes of the box annealing procedure, to become unsatisfactory.
In the case of the approach B, it is difficult to discover the side edge having a larger stress than that of the other side edge of the steel strip because the difference in stress is frequently produced due to unexpected causes. Accordingly, the method of the approach B is not always utilizable for the steel strip coil.